Ultra small core fiber with dispersion tailoring

ABSTRACT

Various embodiments of optical fiber designs and fabrication processes for ultra small core fibers (USCF) are disclosed. In some embodiments, the USCF includes a core that is at least partially surrounded by a region comprising first features. The USCF further includes a second region at least partially surrounding the first region. The second region includes second features. In an embodiment, the first features are smaller than the second features, and the second features have a filling fraction greater than about 90 percent. The first features and/or the second features may include air holes. Embodiments of the USCF may provide dispersion tailoring. Embodiments of the USCF may be used with nonlinear optical devices configured to provide, for example, a frequency comb or a supercontinuum.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/448,003, entitled “ULTRA SMALL CORE FIBER WITH DISPERSIONTAILORING,” filed Apr. 16, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/407,663, filed Mar. 19, 2009, entitled “ULTRASMALL CORE FIBER WITH DISPERSION TAILORING,” now U.S. Pat. No.8,165,441, which claims priority to U.S. Provisional Patent ApplicationNo. 61/039,717, filed Mar. 26, 2008, entitled “ULTRA SMALL CORE FIBERWITH DISPERSION TAILORING;” each of the foregoing applications andpatent is hereby incorporated by reference herein in its entirety.

BACKGROUND

1. Field

Embodiments disclosed herein relate generally to optical fibers and moreparticularly to ultra small core fibers.

2. Description of the Related Art

Ultra small core fibers (“USCFs”) have a variety of applicationsespecially in devices that utilize optical nonlinearities. Ultra smallcore fibers have been used for supercontinuum generation, wavelengthconversion, soliton-based pulse compression, and so forth.

U.S. Pat. No. 6,792,188 discloses a design where an inner layer of smallholes is used to achieve tailored dispersion of a photonic crystalfiber. In this disclosure, a large number of air holes are used beyondthe inner layer of small air holes. A significant drawback of thisdesign is the need to use a large number of air holes beyond the innerlayer of air holes to reduce confinement loss, especially for corediameters less than 2 μm.

In a paper by J. K. Ranka, et al., “Optical Properties of High-Delta AirSilica Microstructure Optical Fibers,” Optics Letters, vol. 25, pp796-798, 2000, the authors disclose a fiber with a core diameter of 1.7μm surrounded by a triangular arrangement of a large number of air holeswith d/Λ≈0.9, where d is the diameter of an air hole and A is thecenter-to-center spacing of the air holes. The need for low confinementloss leads to the large air hole size (relative to the hole spacing) andthe large number of air holes. The need for low confinement loss makesdispersion tailoring very difficult for a fixed core diameter.

U.S. Pat. No. 7,266,275 discloses a method of dispersion tailoring for afiber incorporating a partially doped core to raise its refractiveindex. For small core diameters less than 2 μm, glass and air boundaryplays a very significant part in confining light in the core. Arefractive index change of a few percent over part of the core has verylittle impact on fiber dispersion.

SUMMARY

Various embodiments of optical fiber designs and fabrication techniquesfor fibers such as USCFs with low splice loss, tailored dispersion,and/or low scattering loss are provided.

Various embodiments include an optical fiber capable of propagatinglight having a wavelength, the optical fiber comprising a core, a firstregion at least partially surrounding the core, and a second region atleast partially surrounding the first region. The first region comprisesa plurality of first features. The first features have a firstdimension, and the plurality of first features have a first fillingfactor in the first region. The second region comprises a plurality ofsecond features. The second features have a second dimension and theplurality of second features have a second filling factor in the secondregion. The first dimension is less than the second dimension and thesecond filling factor is greater than about 90 percent.

Various embodiments include an optical fiber capable of propagatinglight having a wavelength wherein the optical fiber comprises a core, afirst region at least partially surrounding the core, an air claddingsurrounding the first region, an outer layer surrounding the aircladding, and a plurality of webs mechanically coupling the first regionand the outer layer such that the air cladding is disposed therebetween.The first region comprises a plurality of first features. The firstfeatures have a first dimension and the plurality of first features havea first filling factor in the first region. The air cladding has anair-filling factor greater than about 90%.

Various embodiments include an optical fiber capable of propagatinglight having a wavelength, wherein the optical fiber comprises a core, afirst air cladding at least partially surrounding the core and a secondair cladding at least partially surrounding the first air cladding. Thefirst air cladding comprises a plurality of air holes having a firstsize. The second air cladding comprises a plurality of air holes havinga second size. The second size is greater than the first size. The firstair cladding and the second air cladding are configured so that thefiber dispersion has a zero dispersion wavelength less than thewavelength of the light.

Various embodiments include a non-linear fiber optic system forproducing broadband optical pulses comprising a laser source producingoptical pulses having a wavelength, an optical fiber optically coupledto the laser source and capable of propagating light having saidwavelength, and means for controlling dispersion of the pulses and forsubstantially confining the pulse to the core. The optical fiberreceives energy from the laser source at a peak power. The optical fibercomprises a core having a diameter less than about 4 μm and sufficientlysmall such that the peak power exceeds a threshold for non-linearity ofthe optical fiber. The fiber produces broadband amplified pulses havinga spectral bandwidth of at least about 50 nm.

For purposes of this summary, certain aspects, advantages, and novelfeatures are described. It is to be understood that not necessarily allsuch advantages may be achieved in accordance with any particularembodiment. Thus, for example, those skilled in the art will recognizethat embodiments may be provided or carried out in a manner thatachieves one advantage or group of advantages as taught herein withoutnecessarily achieving other advantages as may be taught or suggestedherein. Furthermore, embodiments may include several novel features, nosingle one of which is solely responsible for the embodiment's desirableattributes or which is essential to practicing the systems and methodsdescribed herein. Additionally, in any method or process disclosedherein, the acts or operations of the method or process may be performedin any suitable sequence and are not necessarily limited to anyparticular disclosed sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cross-section of an embodiment of anoptical fiber. The fiber comprises a core with a diameter 2ρ, and acladding comprising holes with diameter, d, and center-to-centerseparation, Λ.

FIGS. 1A and 1B provide examples of methods to estimate fillingfractions or factors for a finite matrix (or array) of cladding featuresof regular and arbitrary shapes, respectively.

FIG. 2 is a graph that shows an example of simulated results fordispersion of fibers having the cross-section schematically illustratedin FIG. 1. In this example, the fiber cladding comprises six air holeswith a refractive index of 1 and with d/Λ=0.99. The graph showsdispersion (in units of ps/nm/km) as a function of wavelength (in μm)for various core diameters.

FIG. 3A schematically illustrates a cross-section of an embodiment of aUSCF comprising a core surrounded by first features that compriserelatively small holes. The core and the first features are surroundedby second features that comprise relatively larger holes. In someembodiments, the first features may be used to tailor dispersion and thesecond features may be used to provide optical confinement.

FIG. 3B schematically illustrates a length of an embodiment of a USCFthat is spliced to a conventional fiber. This embodiment of the USCFprovides low loss splice due to expansion of an optical mode at thesplice.

FIG. 4 is a graph that shows an example of simulated results fordispersion of an embodiment of a USCF fiber having a cross-section asshown in FIG. 3A. In this simulation, the USCF has a core diameter of1.5 μm, and the first features are circular with diameter d andcenter-to-center spacing Λ. The graph shows dispersion (in units ofps/nm/km) as a function of wavelength (in μm) for various d/Λ of thefirst features.

FIG. 5 is a graph that shows an example of simulated results fordispersion of fibers having a core diameter of 1.25 μm. The graph showsdispersion (in units of ps/nm/km) as a function of wavelength (in μm)for various d/Λ of the inner cladding features.

FIG. 6 is a graph that shows an example of simulated results fordispersion of fibers having a core diameter of 1.0 μm. The graph showsdispersion (in units of ps/nm/km) as a function of wavelength (in μm)for various d/Λ of the inner cladding features.

FIG. 7 schematically illustrates an embodiment of a preform stack formaking canes and a cross section of an embodiment of a fabricated cane.The cane may be drawn into an embodiment of USCF.

FIG. 8 includes scanning electron microscope (SEM) photographs of anembodiment of a USCF drawn using an embodiment of the cane shown in FIG.7. The left panel shows the fiber cross-section, and the right panel isa closeup view of the center regions of the fiber.

FIG. 8A illustrates an example polarization maintaining fiberoscillator-amplifier coupled to a highly nonlinear fiber in conjunctionwith one embodiment of an oscillator phase control system.

FIG. 8B illustrates one embodiment of the polarization maintaining fiberoscillator of FIG. 8A wherein the oscillator design allows for phasecontrol of the oscillator.

FIGS. 8C-8E illustrate some of the possible approaches for controllingthe beat signal related to the carrier envelope offset frequenciesassociated with the system of FIG. 8A.

FIGS. 8F and 8G illustrate other design variations of fiber oscillatorsthat facilitate generation of carrier envelope offset frequency beatsfor precision frequency comb generation.

FIG. 9 is an illustration of an embodiment of a compact fiber basedsource comprising a quasi-phase matched optical parametric amplifier(OPA) that outputs amplified ultra broadband pulses.

FIG. 10A schematically illustrates an example of a co- andcounter-pumped fiber amplifier, which may operate as a non-linearamplifier or power amplifier.

FIG. 10B schematically illustrates an example of a side-pumped fiberamplifier, which may operate as a non-linear amplifier or a poweramplifier.

FIG. 10C shows the spectrum from the nonlinear amplifier as a functionof pump diode current and ASE spectral output at peak current.

FIG. 10D schematically shows the spectrum of pulses with self-phasemodulation propagating in a positive dispersion fiber.

These and other features will now be described with reference to thedrawings summarized above. The drawings and the associated descriptionsare provided to illustrate embodiments of the invention and not to limitthe scope of the invention. Throughout the drawings, reference numbersmay be reused to indicate correspondence between referenced elements. Inaddition, the first digit of each reference number generally indicatesthe figure in which the element first appears.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to “fillingfraction,” also known as “void-filling fraction” or “air-filling”fraction. In certain embodiments, the filling fraction refers to across-sectional area occupied by certain features in a region (such as,e.g., air holes in a cladding) divided by a total cross-sectional areaof the region. The term filing factor is also used herein synonymouslywith filling fraction.

In certain such embodiments, the filling fraction may be determined forperiodic features in the region, where, for example, a basic unit cellis repeated to fill the region. One possible expression for the fillingfraction for circular features having a diameter d and center-to-centerspacing (pitch) A in an infinite triangular matrix was disclosed in U.S.Pat. No. 6,444,133, entitled “Method of Making Photonic Bandgap Fibers”as the following formula:

${{Filling}\mspace{14mu} {Fraction}} = {\frac{\pi}{2\sqrt{3}}\left( \frac{d}{\Lambda} \right)^{2}}$

This formula for the filling fraction may be applied to, for example,conventional photonic bandgap or photonic crystal fibers having a verylarge number of features (such as holes). In many such fibers, at leasta portion of the holes are circular in shape.

For the purpose of illustrating the filling factor for variousembodiments, it is instructive to provide estimates for a fiber having alimited number of features and/or a finite matrix of features. Featureshaving various size and shapes may be utilized in various embodiments,for example, and the features may have regular and/or irregular shapes.In some embodiments, estimates for the filling fraction may not beobtainable in closed form, and may be determined using computationalnumerical methods.

In certain fiber embodiments, the fiber cross-section comprises aregular matrix of substantially identical features that aresymmetrically arranged. A first example of a fiber cross-section havinga finite number of features is shown in FIG. 1A. In this example, anannular region (or band) 1510 between concentric circles 1501 and 1502includes a matrix of twelve features 1500. In this example, each feature1500 of the matrix has substantially the same cross-sectional shape andsize and has a cross-sectional area A_(F). The feature area A_(F) maybe, for example, computed from predetermined geometric parametersdescribing the shape and size, digitized using a graphic tool, and/orestimated with any suitable computational methods and means. In theexample shown in FIG. 1A, an inner radius R₁ of the annular region 1510corresponds to the inscribed circle 1501, which is tangent to inneredges of the features 1500. An outer radius R₂ of the annular region1510 corresponds to the circumscribed circle 1502, which is tangent toouter edges of the features 1500. The filling fraction may be estimatedby calculating the ratio of the total area of the features 1500 to thearea of the annular region 1510. In the example shown in FIG. 1A, thefilling fraction may be determined to be 12A_(F)/π(R₂ ²−R₁ ²).

In other embodiments, the fiber cross-section may comprise features thatare irregularly shaped and/or asymmetrically arranged. In a secondexample schematically illustrated in FIG. 1B, the fiber cross-sectioncomprises features 1600 that are disposed in a region (or band) 1610having an inner boundary 1601 and an outer boundary 1602. In thisexample, each of the boundaries 1601, 1602 comprises tangent lineslinking adjacent features. The boundaries 1601, 1602 may include aportion of the edge of a feature 1600 in some cases. As can be seen fromthe example in FIG. 1B, the inner boundary 1601 comprises line segments.In some cases, lines linking adjacent features intersect before reachingan edge of the feature (see, e.g., the inner boundary near features1604-A and 1604-B). If line segments from adjacent features intersectthe edge of a feature before intersecting each other, then a portion ofthe edge of the feature will form part of the boundary (see, e.g.,feature 1605 where edge portion 1605-A forms part of the boundary 1602).

FIG. 1B demonstrates that portions of the boundaries of the region 1610may be convex or concave. For example, in FIG. 1B, the boundary 1601 isconvex and a portion 1606 of the boundary 1602 is concave. As can beseen from FIG. 1B, a boundary may not intersect all the features. Forexample, the inner boundary 1601 does not intersect the features 1604-Aor 1604-B. Note that in both these embodiments, for a given portion ofthe region (or band) 1610 between two features, the region (or band) isat least as thick as the smallest of the two features. Nowhere is theregion or band 1610 thinner than the two closest features that definethe region or band.

The above-described methods for determining the filling fraction may beused for fiber embodiments having cross-sections with multiple regionsof features. Estimates obtainable using mathematical formulas, numericalcomputations (including manual estimates) are sufficiently accurate soas to not substantially affect predicted optical propagation propertiesof embodiments of USCFs.

Three problems have hampered practical develoμment of state-of-artUSCFs. The first problem is the difficulty in splicing a USCF to aconventional fiber due to the large mode size mismatch of the USCF andthe conventional fibers. For example, a USCF can have a mode fielddiameter (MFD) of less than 3 μm, while conventional fibers typicallyhave MFD larger than 6 μm. The second problem is the difficulty oftailoring fiber dispersion while maintaining a low confinement loss.Many USCF fibers have a cladding formed by air holes in a backgroundmaterial, typically a glass. For very small core diameter opticalfibers, very large air holes are required to reduce or minimizeconfinement loss and/or to avoid having an excessively large number ofair holes. This leads to inflexibility for tailoring dispersion of theUSCF. Dispersion tailoring is advantageous for optimized operation ofcertain devices utilizing optical nonlinearities, because of the abilityto phase match and/or group velocity match at different wavelengthsand/or to operate in higher order soliton modes. The third problemconfronting a USCF is high loss. USCF loss arises primarily fromscattering loss at glass-air interfaces. High loss occurs for tworeasons when the core diameter is small. The first reason is that thereis much more optical energy at the glass-air interfaces for small corefibers than for large core fibers. Certain USCF comprise thin glasswebs, which result from using large air holes to reduce confinementloss. The second reason for the scattering loss is that the webs tend tohave more surface irregularities due to their small thickness, whichleads to more scattering loss.

Computer simulations have been performed to calculate the dispersion inoptical fibers. As described above, in some small core fibers with smallcore diameter (<2 μm), very large air holes are used to reduce orminimize confinement loss. To calculate the dispersion of such fibers,the computer simulation uses a fiber design having a cross-section shownin FIG. 1. The fiber of FIG. 1 comprises a cladding having six largecircular holes 1001 having a diameter of d. The center-to-centerseparation of the holes 1001 is A. For the simulation, the holes 1001are assumed to be filled with a material having a refractive index ofone (e.g., air). The fiber comprises a core 1002 having a diameter2ρ=2Λ−d.

Because high optical nonlinearity is achievable with very small coresizes, various embodiments utilize very small core fibers, for example,a fiber having a core of with a diameter in a range from about 1 μm toabout 4 μm. In certain applications, it may be desirable to achieve arelatively high index contrast between the holes 1001 and thesurrounding material in order to guide an optical beam propagatingwithin the fiber. Therefore, in certain preferred embodiments the holes1001 are considered to be filled with a gas or a mixture of gases (e.g.,air), to provide a reasonably high index contrast.

FIG. 2 is a graph that shows an example of simulated results for thedispersion of fibers having the cross-section schematically illustratedin FIG. 1. The graph shows dispersion (in units of ps/nm/km) of thefiber as a function of the wavelength (in μm) of light propagating inthe fiber. The wavelength range shown in FIG. 1 is from 0.5 μm to 2.0μm. In this example, the simulation was performed for d/Λ=0.99. Curves2001-2008 illustrate the dispersion for eight values of the corediameter 2ρ=0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 μm, respectively.The dispersion curves 2001-2008 show few features in this simulation. Azero dispersion wavelength (ZDW) is a wavelength at which the dispersionis zero. The dispersion curves 2001-2008 each exhibit two ZDWs, thefirst ZDW at a smaller wavelength than the second ZDW (the second ZDW isnot shown in FIG. 2 for curves 2006, 2007, and 2008). FIG. 2demonstrates that as the core diameter decreases, the first and thesecond ZDW move toward shorter wavelengths. For wavelengths between thefirst and the second ZDW, the dispersion is positive (e.g., anomalousdispersion). FIG. 2 demonstrates that the maximum of the anomalousdispersion is about 200 ps/nm/km for all fiber core diameters used inthe simulation, with the exception of the fiber with the 0.6 μm corediameter (the curve 2001).

For a variety of nonlinear devices providing, for example,supercontinuum generation and/or wavelength conversion, low dispersionand/or relatively flat dispersion may be advantageous, due to the needfor higher-order soliton effects and phase matching among differentoptical wavelengths in some such devices. Additionally, a small core isgenerally desirable in these fibers, because enhanced nonlinearity canbe achieved through higher optical intensity in a small core fiber at afixed optical power. The results shown in FIG. 2 demonstrate that smallcore fibers having the cross-section shown in FIG. 1 generally do notprovide low and/or relatively flat dispersion. Accordingly, adisadvantage of such USCFs is the inability to simultaneously satisfythe need for both low confinement loss and tailored dispersion withouthaving to use an excessive large number of air holes.

FIG. 3A schematically illustrates an example cross-section of anembodiment of a USCF 3001. The USCF 3001 comprises a core 3002, a firstregion comprising first features 3003, and a second region comprisingsecond features 3004. The first region substantially surrounds the core3002, and the second region substantially surrounds the first region. Anouter layer 3005 substantially surrounds the second region. The core3002 and the outer layer 3005 may be formed from the same material orfrom different materials. In some embodiments, the core 3002 and/or theouter cladding 3005 comprise a glass such as, for example, fused silica.In some embodiments, the core 3001 comprises fused silica doped with oneor a combination of germanium, phosphorous, fluorine, boron, aluminum,titanium, tin, and rare earth elements. For example, in the embodimentillustrated in FIG. 3A, the core 3002 of the fiber 3001 comprises a coreregion 3006 that is doped with a dopant such as, e.g., germanium. Thecore region 3006 may form a central portion of the core 3002 as depictedin FIG. 3A. Embodiments of the fiber 3001 comprising a doped core region3006 may provide enhanced optical nonlinearity and/or reduced spliceloss, for example, as described in U.S. patent application Ser. No.11/691,986, filed Mar. 27, 2007, entitled “Ultra High Numerical ApertureOptical Fibers,” which is owned by the assignee of the presentapplication, and which is hereby incorporated by reference herein in itsentirety. In other embodiments, the glass may comprise an oxide glass, afluoride glass, and/or a chalcogenide glass, any of which may be dopedwith one or more of dopants described above for fused silica. In certainembodiments, at least a portion of the core 3001 is doped to provideoptical gain. In some embodiments, the fiber 3001 may be “all-glass,”such that the core 3002 and the outer layer 3005 in which the first andthe second features 3003, 3004 are disposed may comprises glass at leastover a portion of the fiber length. In some all-glass embodiments, thefirst features 3003 and/or the second features 3004 may comprise aglass.

In the embodiment illustrated in FIG. 3A, the first features 3003 andthe second features 3004 comprise holes that may be at least filled witha material having a refractive index different from the material formingthe core 3002 and/or the outer layer 3005. For example, in someembodiments, the first and the second features 3003, 3004 are filledwith air. In other embodiments, some or all of the first and/or thesecond features 3003, 3004 may be filled with other gases and/orliquids. Generally, a refractive index of approximately unity for thematerial in the features is desirable for increased index contrast. Inyet other embodiments, some or all of the first and/or the secondfeatures 3003, 3004 are evacuated to provide a partial vacuum.

In the embodiment of the fiber 3001 schematically illustrated in FIG.3A, the first features 3003 comprise six substantially circular holesthat are arranged substantially symmetrically around the core 3002. Inother embodiments, a different number of first features may be utilizedsuch as, for example, 1, 2, 3, 4, 5, 10, or more features. In otherembodiments, the first features 3003 may be arranged differently thanshown in FIG. 3A and/or may have different shapes (e.g., non-circular)than shown in FIG. 3A. For example, the first features 3003 are notsymmetrically arranged around the core 3002 in some embodiments.

In the embodiment of the fiber 3001 schematically illustrated in FIG.3A, the second features 3004 comprise twelve holes that are arrangedsubstantially symmetrically around the core 3002 and around the firstfeatures 3003. In other embodiments, a different number of secondfeatures may be utilized such as, for example, 1, 2, 3, 4, 5, 10, 15,24, or more features. In other embodiments, the second features 3004 maybe arranged differently than shown in FIG. 3A. For example, the secondfeatures 3004 are not symmetrically arranged around the core 3002 and/orthe first features 3003 in some embodiments. In the embodiment shown inFIG. 3A, the second features 3004 comprise radially elongated holeshaving a “teardrop” shape. Other shapes are used in other embodiments.In some embodiments, adjacent second features 3004 are disposedrelatively close to each other, thereby forming a relatively thin,elongated web 3010 between the adjacent features. For example, in theembodiment shown in FIG. 3A, the fiber 3001 comprises 12 webs 3010. Theratio of radial length to transverse width may be equal to or greaterthan about 4, about 6, about 8, about 10 or more in some embodiments.

In the embodiment of the fiber 3001 depicted in FIG. 3A, the firstfeatures 3003 have a diameter A. The second features 3004 have a radiallength R and an azimuthal length L. In the illustrated embodiment, thediameter A of the first features 3003 is less than the radial length Rand the azimuthal length L of the second features 3004. The radiallength is measured along the centerline through second features 3004 inthe radial direction, and the azimuthal (e.g. arc) length L is measuredthrough the azimuthally directed line midway along the radial length ofthe second features 3004. For example, in some embodiments, the ratioR/A (and/or the ratio L/A) may be in a range from about 1 to about 50.In certain embodiments, the ratio of R/A (and/or L/A) is equal to orgreater than about 1, about 2, about 3, about 4, or about 5.Additionally, the ratio R/L is equal to or greater than about 2.Different ratios are used in other embodiments. As discussed above,first region comprises the first features 3003, and the second regioncomprises the second features 3004. In certain embodiments, the firstfeatures 3003 have a first filling fraction in the first region, and thesecond features 3004 have a second filling fraction in the secondregion. The first filling fraction and the second filling fraction mayhave any suitable values. For example, in certain embodiments the firstfilling fraction is between about 0.2% and about 90%. The first fillingfraction is greater than about 30% in some embodiments. In otherembodiments, the first filling factor is greater than about 5%, greaterthan about 15%, greater than about 25%, greater than about 35%, greaterthan about 45%, greater than about 55%, or some other value. In certainembodiments the second filling fraction is between about 90% and about99.9%. The second filling fraction is greater than about 90% in someembodiments. In other embodiments, the second filling factor is greaterthan about 50%, greater than about 60%, greater than about 70%, greaterthan about 80%, greater than about 95%, or some other value. Forexample, in the embodiment of the USCF 3001 schematically illustrated inFIG. 3B, the first filling factor is about 55% and the second fillingfactor is about 95%.

In some embodiments of the fiber 3001, the first features 3003 may beused to tailor dispersion, and the second features 3004 may be used toprovide optical confinement for light propagating in the fiber 3001(e.g., to reduce confinement loss). For example, in certain embodiments,the size of the first features 3003 may be used for dispersiontailoring, and the radial and/or azimuthal size of the second features3004 may be used for reducing confinement loss.

In some embodiments, the core 3002 may have a size that is aboutone-half the wavelength λ of the light propagating in the fiber 3001.Fiber embodiments with core sizes as small as λ/2 may provide reasonableconfinement loss and a range of tailored dispersion. An advantage ofsome embodiments of the fiber 3001 is that confinement of optical powerby the first features 3003 reduces the amount of optical power at theinterfaces of the second features 3004 (e.g., air-glass interfaces insome embodiments). As described above, in certain embodiments, thearrangement of the second features 3004 may form a plurality of webs3010. In certain such embodiments, the webs 3010 may have a relativelyhigh surface area that may include surface irregularities, which couldpossibly contribute toward a higher scattering loss. An advantage ofsome embodiments of the fiber 3001 is that the reduction of opticalpower (by the first features 3003) in the region of the second features3004 also may reduce scattering losses at the web interfaces, therebyeffectively reducing the scattering loss of the fiber 3001.

Another possible benefit of some embodiments of the fiber 3001 is thatthe fiber 3001 may be spliced to a conventional fiber with relativelylow splice loss. The conventional fiber may a step-index fiber, agraded-index fiber, or any other suitable optical fiber. In someembodiments, the fiber 3001 may be spliced to a holey fiber, photoniccrystal fiber, or a length of fiber that is substantially similar to thefiber 3001.

FIG. 3B schematically illustrates an embodiment of the fiber 3001coupled at a splice 3200 to a conventional fiber 3100. In thisillustrative example, the conventional fiber 3100 is a high numericalaperture fiber comprising a core 3101 that is larger than the core 3002of the fiber 3001. The fiber 3100 supports propagation of an opticalmode 3102. While propagating in the fiber 3100, the optical mode 3102 isconfined substantially to the core as indicated by the curve 3102 inFIG. 3B, which schematically represents a modal energy distribution.

The splice 3200 may be produced by any suitable splicing technique suchas, for example, a fusion splice. For example, in one embodiment of amethod for splicing the fiber 3001 to the fiber 3100, a section of thefiber 3001 is heated (e.g., by an electric arc) before splicing in orderto reduce or substantially eliminate the first features 3003. Afterheating, the section may have a reduced cross-sectional area in someembodiments. In certain embodiments of this method, some of the heatedsection of the fiber 3001 fuses, melts, or collapses into asubstantially solid structure with substantially total elimination ofthe first features 3003 (possibly due to the larger surface tension ofsmall air holes in some embodiments). In certain embodiments, the secondfeatures 3004 are reduced in size by the heating, but the secondfeatures near the splice region (indicated by reference numeral 3202)are not fully collapsed (see, FIG. 3B). In other embodiments, the secondfeatures 3004 are substantially fully collapsed near the splice 3200. Insome embodiments of the splicing method, the core 3002 of the USCF 3001remains substantially intact during the heating/collapsing/splicingprocess. For example, in the embodiment illustrated in FIG. 3B, theoptional doped region 3006 of the core 3002 remains intact indicated byreference numeral 3201) near the splice 3200. The optional doped region3201 near the splice 3200 may further confine optical power in the core3201 and lower splice loss at the splice 3200.

FIG. 3B schematically illustrates propagation of an optical mode 3007 inthe USCF 3001 toward the splice 3200. While propagating in the USCF3001, the mode 3007 is confined substantially to the core 3001. As theoptical mode 3007 nears the region where the splicing process hasreduced (or substantially eliminated) the first features 3003, the mode3007 begins to expand as schematically illustrated in FIG. 3B. Incertain embodiments, expansion of the mode 3007 may cause negligibleloss if an adiabatic condition is satisfied near the splice 3200, e.g.,if the reduction of the first features 3003 occurs more slowly than whata local optical mode can follow. In the illustrated embodiment, when themode 3007 reaches the splice 3200, the optical power of the mode 3007substantially matches the optical power of the mode 3102 that can bepropagated in the core of the conventional fiber 3100. For example, theabove-described method advantageously permits a low loss splice to beperformed between the USCF 3001 and the conventional optical fiber 3100,due to the substantially close match of the mode field diameters at thesplice 3200.

FIG. 4 is a graph that shows an example of simulated results fordispersion of an embodiment of a USCF fiber having the cross-sectionshown in FIG. 3A. In this simulation, the USCF has a core diameter2ρ=1.5 μm, and the first features are circular with diameter, d, andcenter-to-center spacing, Λ. The graph shows dispersion (in units ofps/nm/km) as a function of wavelength (in μm) for various d/Λ. Thewavelength range in FIG. 4 is between 0.6 μm and 1.6 μm. Curves4001-4009 are for d/Λ=0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and 0.1,respectively. The results shown in FIG. 4 demonstrate that a range ofdispersion may be achieved for this embodiment of a USCF having a corediameter of 1.5 μm. For example, a relatively flat, low dispersion canbe achieved near a wavelength of 1 μm for USCF embodiments having d/Λbetween about 0.4 and about 0.6. The dispersion curve 4004 has two zerodispersion wavelengths (ZDW), a first ZDW at a shorter wavelength (about0.75 μm), and a second ZDW at a longer wavelength (about 1.56 μm).Curves 4001, 4002, 4003, 4005, 4006, 4007, 4008, and 4009 have a singleZDW in the simulated wavelength range shown in FIG. 4. In some nonlinearsystems such as, for example, systems providing supercontinuumgeneration, it may be advantageous to pump the system at the anomalousdispersion side of the longer wavelength ZDW. Because many convenientpump sources emit light at wavelengths of about 1.05 it may bebeneficial to tailor the dispersion of the USCF so that the first ZDWoccurs at wavelengths slightly shorter than about 1.05 FIG. 4demonstrates that the dispersion of embodiments of the USCF disclosedherein may be tailored so that the first ZDW occurs in a range fromabout 0.7 μm to about 0.87 μm, and such USCF embodiments may beadvantageously used in systems providing supercontinuum generation.

FIG. 5 is a graph that shows another example of simulated results fordispersion of an embodiment of a USCF fiber having the cross-sectionshown in FIG. 3A. In this example, the USCF has a core diameter 2ρ=1.25μm, and the graph shows the dispersion for three values of d/Λ. Curves5001, 5002, and 5003 are for USCF embodiments having d/Λ=0.8, 0.6, and0.4, respectively. FIG. 5 shows for these USCF embodiments that arelatively flat dispersion may be provided, and the dispersion may betailored so that the zero dispersion wavelengths occur in desiredwavelength regions. For example, the first ZDW for the curve 5003 is atabout 0.94 microns, and the dispersion is relatively flat between about1 micron and about 1.25 microns.

FIG. 6 is a graph that shows another example of simulated results fordispersion of an embodiment of a USCF fiber having the cross-sectionshown in FIG. 3A. In this example, the USCF has a core diameter 2ρ=1.0μm, and the graph shows the dispersion for eight values of d/Λ. Curves6001-6008 are for d/Λ=0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, and 0.2,respectively. FIG. 6 demonstrates that certain embodiments of the USCFmay provide tailored positive dispersion, negative dispersion, and/orflattened dispersion in various portions of the wavelength range shownin the graph.

In certain implementations, embodiments of USCF may be fabricatedaccording to a method in which a preform stack is formed into a cane andthe cane is drawn into an optical fiber. FIG. 7 schematicallyillustrates an embodiment of a preform stack for making canes and across-section of an embodiment of a fabricated cane. In the exampleshown in FIG. 7, a rod 7001 having a diameter of about 1.19 mm wasdisposed substantially in the center of a preform stack. The rod 7001was solid and included a germanium-doped silica (SiO₂) center portionsurrounded by a silica cladding. Six tubes 7002 were disposed around therod 7001. Each of the tubes 7002 was hollow and had an inner diameter ofabout 0.61 mm and an outer diameter of about 1.19 mm. Twelve tubes 7003were disposed around the tubes 7002 as shown in FIG. 7. Each of thetubes 7003 was hollow had an inner diameter of about 0.76 mm and anouter diameter of about 1.19 mm. In other embodiments, some or all ofthe tubes 7002, 7003 may be partially or completely solid.

The stack of the rod 7001, the tubes 7002, 7003 was disposed in an outertube 7004. In this example arrangement, the outer tube 7004 comprisedsilica and had an inside diameter of about 6.2 mm and an outsidediameter of about 8.4 mm. In this example, a pressurizing system was useto apply a pressure of about 2 psi to the inside of the tubes 7002 and7003 during the caning process. While the tubes were pressurized, a cane7100 having a 1.63 mm outer diameter was drawn. In certain embodiments,one or more inert gases are used in the pressurization system, while inother embodiments, air, nitrogen, oxygen, and/or other gases can beused.

The cane 7100 was inserted into an outer tube having an inner diameterof about 2 mm and an outer diameter of about 16.33 mm. In this examplefabrication process, a pressure of about 2 psi was applied to the hollowtubes, and a partial vacuum of about −5 in Hg was applied to the insideof the outer tube. While pressurized, the preform was drawn at atemperature of about 195° C. The preform was fed into the heatingfurnace at about 6 mm/min and was drawn at 92 mm/min into a fiber withan outer diameter of about 125 μm.

FIG. 8 includes scanning electron microscope (SEM) photographs of anembodiment of USCF 8001 drawn with the cane 7100 shown in FIG. 7. Theleft panel of FIG. 8 shows the cross section of the fiber 8001, and theright panel is a closeup view showing the inner structure 8002 of thefiber 8001, which comprises a core 8002, first features 8003, and secondfeatures 8004. In this embodiment of the fiber 8001, the first features8003 comprise six substantially circular holes, and the second features8004 comprise twelve “teardrop” shaped holes. FIG. 8 demonstrates thatin this example embodiment, the second features 8004 are significantlyexpanded under pressure during fiber drawing to achieve a largerair-filing factor than possible with certain conventional stack-and-drawprocesses such as, for example, the process described in U.S. Pat. No.6,792,188.

This fabrication procedure described herein is one possible embodimentof the fabrication procedure. In other embodiments of the procedure,variations of rod and tube dimensions, applied pressure, applied vacuum,and drawing condition may be used. In other embodiments, additionallayers of tubes may be used to provide third features, fourth features,and so forth. Different numbers of tubes may be used. Further details offiber fabrication procedures are described in, for example, theabove-incorporated U.S. patent application Ser. No. 11/691,986.

Example Applications for USCF Embodiments

Embodiments of the USCFs described herein advantageously may be utilizedin a variety of applications including, for example, nonlinearamplifiers, continuum generation, frequency metrology systems employingcomb generators, and systems for stretching ultrashort pulses.

The disclosure of U.S. patent application Ser. No. 11/372,859, entitled“Pulsed Laser Sources,” filed Mar. 10, 2006 (hereinafter the '859application), and published as U.S. Patent Application Publication2006/0198398 is hereby incorporated by reference in its entirety. FIGS.8A-8G (reproduced herein as FIGS. 8A-8G) and the corresponding text ofthe '859 application describe embodiments of systems for frequency combgeneration, which may be used for frequency metrology and/or highresolution spectroscopy. Embodiments of such systems may be useable forprecision timing measurements and/or for high resolution spectroscopy,the latter application showing promise for disease detection.

By way of example, FIG. 8A of the '859 application illustrates anembodiment of a polarization maintaining fiber oscillator-amplifiercoupled to a highly nonlinear fiber in conjunction with one embodimentof an oscillator phase control system. In FIG. 8A, the frequency combsource 800 comprises a polarization maintaining fiber oscillator 801. Anoutput from the oscillator 801 is directed via an isolator 802 to apolarization maintaining fiber amplifier 803. As shown in FIG. 8A, thefiber amplifier 803 is connected via a splice 804 to a highly nonlinearfiber (HNLF) 805. The highly nonlinear fiber 805 is preferablyconstructed from a holey fiber or a standard silica fiber or usingbismuth-oxide based optical glass fiber in various embodiments. Thedispersion of the highly nonlinear fiber 805 is preferably close toapproximately zero at the emission wavelength of the oscillator 801 forcertain designs. Even more preferably, the dispersion profile isflattened, i.e., the third-order dispersion of the fiber 805 is equallyclose to approximately zero. The highly nonlinear fiber 805 does notneed to be polarization maintaining since it is relatively short (on theorder of few cm long), thereby enabling long-term polarization stableoperation. The length of the highly nonlinear fiber 805 is preferablyselected to be less than approximately 20 cm to preserve the coherenceof the generated continuum. Other designs, however, are possible.Embodiments of the USCF may be utilized in embodiments of the frequencycomb source shown in FIG. 8A and may, in various embodiments, extend thecontinuum, reduce pump requirements, and/or provide for higher degree ofcoherence.

The (continuum) output from the highly nonlinear fiber 805 is injectedvia a splice 806 to a wavelength division multiplexing coupler 807. Thecoupler 807 directs the long and short wavelength components from thecontinuum to a long wavelength coupler arm 808 and a short wavelengthcoupler arm 809 respectively. The long wavelength components aresubsequently frequency doubled using exemplary lenses 810, 811, 812,813, as well as a doubling crystal 814. After frequency doubling theresulting output preferably has a substantially same wavelength as atleast part of the short wavelength components traveling in the arm 809.Additional optical elements 815 and 816 can be inserted into the beampaths of the arms 808 and 809 for spectral filtering, optical delayadjustment, as well as polarization control. Spectral filtering elementsare selected to maximize the spectral overlap of the signals propagatingin arms 808 and 809. As another example, the optical element 815 cancomprise appropriate wave-plates that control the polarization state ofthe light in front of the doubling crystal 814.

The frequency-doubled light from the arm 808 and the light from the arm809 are subsequently combined in a polarization-maintaining coupler 817which preferably has a 50/50 splitting ratio. The beat signal frominterference of the two beams injected into the coupler 817 is detectedby a detector 818.

As shown in FIG. 8A, one selected harmonic of the beat signal atfrequency f_(n,m,beat) may be directed via an electrical feedbackcircuitry 819 to the oscillator 801.

An optical element 816 a may be inserted in an optical path after thetwo arms 808, 809 are combined. The optical elements 816 and 816 a thatcan be inserted into the arm 809 and in the combined signal arm beforethe detector 818 may comprise a narrow bandpass filter that narrows thespectral width of the signal transmitted through the arm 809.

To produce an optical output of the frequency comb source which is used,for example, for a frequency metrology experiment, part of the frequencycomb can be coupled out from a location 818 b after the highly nonlinearfiber or from a location 818 a after the coupler 817 and interferometer.The optical output can also be coupled out at a location 818 d after theoscillator or at a location 818 c after the amplifier, if for exampleonly the spectral part of the oscillator or amplifier bandwidth of thecomb is desired.

FIG. 8B illustrates one possible embodiment of the oscillator 801described above in reference to FIG. 8A. The oscillator 801 includes asaturable absorber module 820 comprising collimation and focusing lenses821 and 822 respectively. The saturable absorber module 820 furthercomprises a saturable absorber 823 that is preferably mounted onto afirst piezo-electric transducer 824. The first piezo-electric transducer824 can be modulated to control, for example, the repetition rate of theoscillator 801.

The oscillator 801 further comprises an oscillator fiber 825 that ispreferably coiled onto a second piezo-electric transducer 826. Thesecond piezo-electric transducer 826 can be modulated for repetitionrate control of the oscillator 801. The oscillator fiber 825 ispreferably polarization-maintaining and has a positive dispersionalthough the designs should not be so limited. The dispersion of theoscillator cavity can be compensated by a fiber grating 827 whichpreferably has a negative dispersion and is also used for outputcoupling. It will be understood that a positive dispersion fiber gratingand a negative dispersion cavity fiber may also be implemented.Furthermore, the fiber grating 827 can be polarization-maintaining ornon-polarization-maintaining.

The pump light for the oscillator 801 can be directed via apolarization-maintaining wavelength division multiplexing coupler 828from a coupler arm 829 attached to a preferably single-mode pump diode830.

FIGS. 8C-D illustrate some of approaches to using the beat signalfrequency to control repetition rate as well as carrier envelope offsetfrequency of the frequency comb source 800. As shown in FIG. 8C, a pumpcurrent 840 can be changed, wherein a change in the pump current cancause a change of the beat signal frequency and more particularly thecarrier envelope offset frequency.

As shown in FIGS. 8D and 8E, the absolute position of the carrierenvelope offset frequency can be controlled by adjusting the temperatureof the fiber grating 827 with a heating element 842. Alternatively,pressure applied to the fiber grating 827 can also be used to set thecarrier envelope offset frequency using for example a piezo-electrictransducer 844.

FIGS. 8A and 8B describe the basic design of a frequency comb sourcebased on a low noise phase-locked fiber laser for frequency metrology.Modifications to this basic design can be easily implemented asdescribed below.

FIG. 8F illustrates one embodiment of a fiber based continuum source 850where the amplifier (803 in FIG. 8A) is omitted. In the exemplarycontinuum source 850, high quality sub-200 fs pulses are preferablyinjected into a highly nonlinear fiber 854 (805 in FIG. 8A). To generatesuch short pulses, the oscillator-only continuum source 850 preferablygenerates positively chirped pulses in the oscillator 801, which arecompressed in an appropriate length of a negative dispersion fiber 852before injection into the highly nonlinear fiber 854. For theoscillator-only continuum source 850, the amplifier is thus substitutedwith the negative dispersion fiber 852.

As shown in FIG. 8F, the oscillator-only continuum source 850 furthercomprises an interferometer 856 that interferes the two frequencycomponents as described above. The interferometer 856 may be similar tothe two-arm interferometer shown in FIG. 8A (fiber based or equivalentbulk optics components), or may be similar to a one-arm interferometerdescribed below. The output of the interferometer 856 can be detected bya detector 858, and selected signals from the detector 858 can be usedfor feedback control 860 in a manner similar to that described above inreference to FIGS. 8A-E.

FIG. 8G illustrates an example one-arm interferometer 870. Such aninterferometer can be obtained by removing one of the arms (arm 809 inFIG. 8A) and modifying the remaining arm. As shown in FIG. 8G, theinterferometer 870 comprises a group delay compensator 872 inline with adoubling crystal 874. The group delay compensator 872 receives acontinuum signal from a highly nonlinear fiber located upstream, andensures that the frequency doubled and non-doubled spectral componentsfrom the continuum that are output from doubling crystal overlap intime. Moreover, since the doubled and non-doubled spectral componentsare selected to overlap in optical frequency, these components interfereand the interference signal is detected with a detector downstream.

Highly non-linear fibers corresponding to embodiments of the presentdisclosure may be utilized for supercontinuum generation and may providefor extremely broad bandwidths. The disclosure of U.S. patentapplication Ser. No. 11/091,015, entitled “Optical parametricamplification, optical parametric generation, and optical pumping inoptical fibers systems,” filed Mar. 25, 2005 (hereinafter the '019application), and published as U.S. Patent Publication 2005/0238070 ishereby incorporated by reference in its entirety. Embodiments mayprovide a broad spectral bandwidth for continuum or supercontinuumgeneration. For example, in various embodiments, the bandwidth may be atleast about 50 nm, and in some embodiments at least about 200 nm. Aspectral bandwidth of up to about 1 μm may be generated with embodimentsof highly non-linear USCFs. For example, in some embodiments,supercontinuum from about 0.4 μm to greater than about 1.6 lam may begenerated.

FIG. 4 of the '015 application (reproduced herein as FIG. 9) and thecorresponding text of the '015 application, for example, describe anembodiment of an amplification system comprising a short-pulse fiberlaser 101 whose output is split into two arms by a beam splitter 220. Ina one arm is an optical parametric amplification (OPA) pump 200 thatprovides pump power. The OPA pump 200 outputs high-energy,narrow-bandwidth, pump pulses. In another arm, a broadband continuum isgenerated in a continuum fiber 210. This continuum fiber 210 maycomprise, for example, a fiber having nonlinear properties. Output fromthe continuum fiber 210 is passed through a filter 240 to filter outtwice the center wavelength of the light generated by an OPA pump 200located in a second arm. The filter 240 may pass long- and/orshort-wavelength parts relative to twice the center wavelength of theOPA pump 200. This broadband continuum output comprises a seed pulse forseeding the OPA process.

Accordingly, the output from the continuum fiber 210 after beingfiltered by the filter 240 as well as the pump output from the OPA pump200 are combined by a beamsplitter/coupler 250 and applied to theparametric amplifier 260. The beam splitter 250 thus combineshigh-energy narrow-bandwidth pump pulses from the OPA pump 200 andwide-bandwidth seed pulses from the continuum fiber 210. An amplifiedsignal is produced by the parametric amplifier 260. This amplifiedsignal is applied to the pulse compressor 270.

The fiber laser 101 may be a mode-locked oscillator or a mode-lockedoscillator followed by one or more fiber amplifiers. The fiber laser 101is constructed to deliver pulse energies and peak powers sufficient toproduce a wide enough continuum in the continuum fiber 210, e.g., a fewnanojoules (nJ). For additional background, see, e.g., U.S. PatentPublication 2004/0213302 entitled “Pulsed Laser Sources” filed byFermann et al, which is incorporated herein by reference in itsentirety. In various embodiments, the fiber laser 101 is an Er fiberlaser that produces short optical pulses at about 1560 nm with therepetition rate of 20-100 megahertz (MHz). The laser 101 may producelinearly-polarized light as for example can be obtained by usingpolarization-maintaining (PM) components. The laser is optionallyimplemented as a master-oscillator-power-amplifier (MOPA) configuration.Such lasers are described in, e.g., U.S. Patent Application No.60/519,447, which is incorporated herein by reference in its entiretyand available from IMRA America, Ann Arbor Mich.

In the embodiment schematically illustrated in FIG. 9, the ultrabroadband continuum in one arm is generated in the continuum fiber 210,which may comprise a micro-structured fiber and/or a conventionalsolid-core high-nonlinearity fiber. Optionally, two or more differentnonlinear fiber types can be used sequentially as discussed in U.S.Patent Publication 2004/0213302, which is incorporated herein byreference in its entirety. With such an approach, continuum generationcan be optimized for different spectral parts, thereby resulting instable operation over a wide ultra broadband spectrum.

Alternatively, the output from the splitter 220 can be split into two ormore arms and different nonlinear fibers or sequences of nonlinearfibers in different arms can be used to optimize the continuum outputfor each individual arm. The optimization of the continuum output ineach arm is particularly useful when creating ultra broadband continuaor ultra-flat continua as well as low noise continua. Flat continua arepreferred in most applications to reduce or avoid the occurrence of‘spectral holes’. For example, in optical coherence tomography, spectralholes limit the optical resolution. Equally, in spectroscopy, spectralholes limit the signal/noise of a potential detection system in certainparts of the spectrum, which is generally undesired.

Embodiments of the ultra-small core non-linear fibers disclosed hereinmay be utilized in embodiments of supercontinuum generation systemsdescribed in the '015 application, or in variations thereof. Spectralwidths of at least several hundred nm may be generated in someembodiments.

The disclosures of U.S. patent application Ser. No. 10/437,059 entitled“Inexpensive variable rep-rate source for high-energy ultrafast lasers,”filed May 14, 2003 (hereinafter the '059 application), published as U.S.Patent Application Publication 2004/0240037, and U.S. patent applicationSer. No. 10/813,163, entitled “Modular fiber-based chirped pulseamplification system,” filed Mar. 31, 2004 (hereinafter the '163application), and published as U.S. Patent Application Publication2005/0226286 generally relate to fiber based ultrashort systems. Variousembodiments include non-linear amplifiers for amplifying pulses andconfigurations for pulse stretching and spectral broadening. Thedisclosures of the '059 application and the '163 application are bothincorporated by reference herein in their entirety.

One application of embodiments of USCF is for stretching ultrashortpulses. In some embodiments, it is desirable to stretch ultrashortpulses to a pulse width of about 1 ns prior to amplification. Thestretching may be carried out in a fiber gain medium to provide bothamplification and spectral broadening, and/or in passive fibers. Incertain embodiments, fibers having normal dispersion (group velocitydispersion, GVD) are utilized in combination with self-phase modulationto produce linear chirped and broadened pulses at wavelengths at or near1 μm.

A doped fiber gain medium provides for non-linear amplification. In the'163 application at least one embodiment comprises a non-linearamplifier module. It is nonlinear due to the fact the pulse is nottemporally stretched so that the amplification takes place with highintensity and thus significant self-phase modulation. FIGS. 1B and 1Cfrom the '163 application are reproduced herein as FIGS. 10A and 10B.Typical amplifier configurations are shown in FIG. 10A (a co-propagatingand counter-propagating pumped arrangement) and FIG. 10B (a side-pumpedarrangement), although the precise configuration can be selected frommany known amplifier designs. The spectrum at the output of thisamplifier is shown in FIG. 6A from the '163 application, which isreproduced herein as FIG. 10C. For higher pump currents the spectralwidth is over 20 nm. Thus in this nonlinear amplifier the spectral widthhas been increased by self-phase modulation by more than a factor of 10,from about 2 nm over 10 times to greater than 20 nm. The amplifier is aYb-doped cladding pumped fiber that is 4 meters long. In FIG. 10B, theamplifier is side-pumped with counterpropagating pumping. Even at thelowest current the spectrum has been broadened by self-phase modulation.At the higher current levels, the spectrum is typical for self-phasemodulation propagating in a fiber with positive dispersion. Comparethese spectra to that shown in FIG. 7 of the '163 application, which isreproduced from Govind P. Agrawal, Nonlinear Fiber Optics (AcademicPress, Inc. New York, 1989), and which is reproduced herein as FIG. 10D.

A nonlinear Yb amplifier with positive dispersion, usable with amplifierembodiments such as those schematically shown in FIGS. 10A and 10B, andwhich has been utilized for pulse amplification of a substantiallyunchirped pulse that is significantly spectrally broadened during theamplification and which can be pulse compressed after amplification isdisclosed in the '059 application. In such systems, highest gain andefficiency are not the predominant concern as in the case of otheramplifiers. The gain of about 100 times in this stage is rather low fora fiber amplifier. One goal is to obtain the highest pulse energy in apulse that can be compressed.

Embodiments of the USCF disclosed herein may be used to further improvethe performance of the above-described ultrashort laser systems, orsimilar systems.

Also, embodiments of the present disclosure may be utilized in awide-range of applications wherein one or more of femtosecond,picosecond, nanosecond, and microsecond pulses are directed to a targetmaterial. Further possible applications of both non-linear stretchersand non-linear amplifiers (comprising USCF embodiments) are generallyfound in material processing and micromachining operations. For example,a material processing system may comprise a fiber system, an opticalsystem to direct the pulses to a material, at least one positioningsystem to position the target material relative to one of more pulses,and a system controller.

A wide variety of other applications, both currently known as well asyet to be discovered, are also possible.

While certain embodiments of the disclosure have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the present inventions. A wide variety ofalternative configurations are also possible. For example, components(e.g., layers) may be added, removed, or rearranged. Similarly,processing and method steps may be added, removed, or reordered.

Accordingly, although certain preferred embodiments and examples havebeen described above, it will be understood by those skilled in the artthat the present invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. In addition, while severalvariations have been shown and described in detail, other modifications,which are within the scope of this invention, will be readily apparentto those of skill in the art based upon this disclosure. It is alsocontemplated that various combinations or sub-combinations of thespecific features and aspects of the embodiments may be made and stillfall within the scope of the invention. It should be understood thatvarious features and aspects of the disclosed embodiments can becombined with, or substituted for, one another in order to form varyingmodes and embodiments. Thus, it is intended that the scope of thepresent invention herein disclosed should not be limited by theparticular disclosed embodiments described above.

What is claimed is:
 1. An optical fiber capable of propagating lighthaving a wavelength, the optical fiber comprising: a core having adiameter less than about 4 μm; a first region at least partiallysurrounding the core, the first region comprising a plurality of firstfeatures collectively having a first filling factor in the first regionthat is less than about 90 percent; and a second region at leastpartially surrounding the first region, the second region comprising aplurality of second features collectively having a second filling factorin the second region that is greater than about 90 percent, wherein theoptical fiber is configured to control dispersion of the light andsubstantially confine the light to the core.
 2. The optical fiber ofclaim 1, wherein the core diameter is in a range from about 1 μm toabout 4 μm.
 3. The optical fiber of claim 1, wherein the first fillingfactor is greater than about 50%.
 4. The optical fiber of claim 1,wherein the first filling factor is less than the second filling factor.5. The optical fiber of claim 1, further comprising: an outer layersurrounding the second region; and a plurality of webs mechanicallycoupling the first region and the outer layer such that the secondregion is disposed therebetween.
 6. The optical fiber of claim 5,wherein the second features comprises air holes.
 7. The optical fiber ofclaim 5, wherein at least one of the plurality of webs is substantiallyradial and has a radial length and a transverse width, the radial lengthgreater than the transverse width.
 8. The optical fiber of claim 7,wherein a ratio of the radial length to the transverse width is greaterthan about
 4. 9. The optical fiber of claim 7, wherein a ratio of theradial length to the transverse width is greater than about
 10. 10. Theoptical fiber of claim 7, wherein a ratio of the radial length to thetransverse width is greater than about 4 and less than about
 10. 11. Theoptical fiber of claim 1, wherein the optical fiber has a dispersiontailored to provide supercontinuum generation in a nonlinear opticaldevice.
 12. The optical fiber of claim 11, wherein the supercontinuum isfrom about 0.4 μm to about 1.6 μm in the nonlinear optical device. 13.The optical fiber of claim 1, wherein at least a portion of the fiber isdoped with a gain medium.
 14. The optical fiber of claim 1, wherein thefiber is configured to provide non-linear amplification of lightpropagating in the fiber.
 15. The optical fiber of claim 1, wherein thefiber is configured to receive optical pump light to pump the gainmedium at a pump wavelength.
 16. A frequency comb source comprising theoptical fiber of claim
 1. 17. A supercontinuum source comprising theoptical fiber of claim
 1. 18. An optical parametric amplifier systemcomprising the optical fiber of claim
 1. 19. A pulse stretchercomprising the optical fiber of claim
 1. 20. A chirped pulseamplification system configured to produce ultrashort laser pulses, thesystem comprising at least one of (i) a non-linear fiber amplifiercomprising the optical fiber of claim 1, at least a portion of the fiberbeing doped with a gain medium, or (ii) the optical fiber accordingclaim 1, the optical fiber being undoped.
 21. A non-linear fiber opticsystem configured to produce broadband optical pulses, the systemcomprising: a laser source configured to produce optical pulses; theoptical fiber of claim 1 optically coupled to the laser source andconfigured to propagate the optical pulses, wherein at least a portionof the optical fiber is doped so as to provide an optical gain mediumfor non-linear amplification of the optical pulses; and an optical pumpconfigured to pump the optical gain medium at a pump wavelength.
 22. Theoptical fiber of claim 1, wherein the dispersion of the fiber ispositive over a wavelength range.
 23. The optical fiber of claim 1,wherein the plurality of second features comprise radially-elongatedholes.
 24. The optical fiber of claim 23, wherein the holes have ateardrop shape.
 25. The optical fiber of claim 1, wherein the pluralityof second features are radially elongated and have a first azimuthalwidth at a first radial distance from the core and a second azimuthalwidth at a second radial distance from the core, the second radialdistance greater than the first radial distance, and the secondazimuthal width greater than the first azimuthal width.
 26. The opticalfiber of claim 1, wherein the plurality of second features have a radiallength and an azimuthal length, and a ratio of the radial length to theazimuthal length is greater than or equal to two.
 27. The optical fiberof claim 1, wherein the plurality of first features have a firstdimension, and the plurality of second features have a radial length andan azimuthal length, and a ratio of the radial length to the firstdimension is greater than about 1 and a ratio of the azimuthal length tothe first dimension is greater than about
 1. 28. The optical fiber ofclaim 1, wherein the plurality of first features have a first dimension,and the plurality of second features have a radial length and anazimuthal length, and a ratio of the radial length to the firstdimension is greater than about 5 and a ratio of the azimuthal length tothe first dimension is greater than about
 5. 29. The optical fiber ofclaim 1, wherein the plurality of first features have a first dimension,and the plurality of second features have a radial length and anazimuthal length, and a ratio of the radial length to the firstdimension is greater than about 1 and less than about 5 and a ratio ofthe azimuthal length to the first dimension is greater than about 1 andless than about 5.